Age-related macular degeneration (AMD) is a leading cause of blindness in Americans 60 years of age and older. There are two forms of AMD: Dry or atrophic and Wet or exudative. However, the Wet form of AMD, which is characterized by choroidal neovascularization (CNV), is associated with more severe vision loss caused primarily by the leakage of fluid and blood from the abnormal vessels. Anti-VEGF therapy has emerged as the treatment of choice for patients with wet AMD. However, some patients never respond to anti-VEGF therapy, while still others stop responding after initial success, highlighting the fact that the pathobiology of wet AMD development and progression is still unknown. FasL is an important protein in maintaining immune privilege in the eye, where it is thought to induce apoptosis of infiltrating inflammatory cells. In fact, several reports indicate that FasL expressed on retinal pigment epithelial cells inhibits choroidal neovascularization by triggering apoptosis o vascular endothelial cells. However, FasL exists as both a membrane bound and soluble protein with potentially opposing functions and it is unclear how the different forms of FasL contribute to the development of CNV. We recently developed a unique knock-in mouse strain in which the FasL metalloproteinase cleavage sites were mutated to prevent cleavage of the membrane-bound protein. In these CS mice, FasL is expressed by the physiologically relevant cell types, but these cells are unable to cleave FasL and therefore, can only express mFasL (termed CS mice). For the first time, these mice have allowed us to study the in vivo function of membrane FasL in the absence of soluble FasL. To better understand the function of the different FasL isoforms in the development of choroidal neovascularization, we used a laser-induced murine model of CNV. Our preliminary data indicate that, in CS mice, either the increased expression of mFasL (or the loss of sFasL) prevents vascular leakage following laser induced-CNV. Spectral domain optical coherence tomography (SD-OCT) and Isolectin-B staining demonstrated no difference in the size of the neovascular lesion between CS mice and wild-type mice. However, fluorescein angiography revealed a significant decrease in vascular leakage in CS mice as compared to wild-type mice. These results indicate that membrane FasL does not prevent the ingrowth of choroidal vessels, as previously documented, but rather plays a critical function in preventing vascular leakage, the primary cause of vision loss in patients with wet AMD. We hypothesize that membrane FasL inhibits vascular leakage in laser-induced CNV through Fas mediated pathways triggered in infiltrating macrophages and/or RPE cells. The goals of this proposal are as follows: (Aim 1) Determine whether the decreased vascular leakage in CS mice is due to increased mFasL and/or a lack of sFasL and whether the decreased leakage coincides with a shift in the M1/M2 phenotype of infiltrating macrophages (Aim 2) Establish whether infiltrating macrophages and/or RPE are the critical Fas+ targets through which mFasL mediates inhibition of vascular leakage.